Generally, the black area starts off in the periphery. In the early stages it will generally be a small black area that when you move your eye it may disappear, but if you keep your eye looking straight ahead it will ALWAYS be visible. (Plus, you'll more than likely notice other symptoms like an increase in floaters and/or flashes of light.) If not treated promptly the black curtain/shadow will grow and get worse so that even eye movement doesn't make the black portion disappear.